How to get Mirrored Vector by Plane on Python
You can use mathutils.geometry.intersect_line_plane to project the point onto the plane. This can be done by intersecting the line defined by the point and the point offset by the plane's normal vector with the plane:

proj = intersect_line_plane(point, point + plane_normal, plane_position, plane_normal)

Once you have the projected point you can use the vector from the original point to the projection to get the mirrored point.

from mathutils.geometry import intersect_line_plane

def get_mirrored_vector(point, plane_position, plane_normal):
    proj = intersect_line_plane(point, point + plane_normal, plane_position, plane_normal)
    mirrored_point = 2 * proj - point
    return mirrored_point